Pulse-width-modulation (PWM) amplifiers, also known as Class-D amplifiers, operate on a similar principle as that of switching power supplies except that the reference voltage of a PWM amplifier is a varying signal instead of a fixed voltage.
Generally, Class-D amplifiers are classified as analog-input Class-D and digital-input Class-D, i.e., fully digital audio amplifier.
Class-D amplifiers are much more power efficient than Class-AB amplifiers. Because of their greater efficiency, Class-D amplifiers require smaller power supplies and eliminate or reduce the need for heat sinks, significantly reducing overall system cost, size and weight. Other benefits include longer battery operation, a quiet and better listening environment, and an integrated audio amplifier with high output power (>20 W/Channel).
A traditional Class-D amplifier requires an output filter, which increases system size and solution cost, limiting its use in portable devices. Filterless Class-D amplifiers eliminate the output filter while keeping the efficiency benefit. The filterless modulation scheme brings Class-D amplifiers approximately equal to Class-AB amplifiers in cost and size, but with great efficiency advantages.
One method of achieving filterless Class-D operation, increased efficiency, and reduced cost is to deliver current to the load only when needed, and once delivered, maintain the current, thereby not decaying or wasting energy in removing the current from the load when no input signal is delivered. One such methodology is a quaternary modulation scheme that has four states of operation. The modulation scheme uses the four states to drive a load such as a speaker, depending on the audio input signal. This quaternary scheme is described in detail, for example, in U.S. Pat. No. 6,262,632 to Corsi et al., the entirety of which is hereby incorporated by reference herein.
Eliminating the filter causes the Class-D amplifier to radiate electromagnetic interference (EMI). This EMI phenomenon is also addressed by Score et al. in U.S. Pat. No. 6,614,297, the entirety of which is hereby incorporated by reference herein. Score et al. describes a system where ternary PWM coding is used rather than traditional binary PWM coding or quaternary PWM coding. EMI is improved as ΔV for the ternary PWM is |VDD| whereas it is |2VDD| for binary PWM. Although ternary PWM coding can be achieved by quaternary switching behavior, the common-mode EMI component of ternary PWM coding by quaternary switching behavior is larger than by ternary switching behavior.
While the modulation scheme and amplifier of Score et al. improve EMI performance, as well as efficiency at small inputs with filterless operation, the technique of Score et al. require a logic module to encode the 4-state switching signal (quaternary) to three state switching signal (ternary) in order to realize these advantages.
An improved modulation scheme is desired for ternary PWM coding generation for Class-D amplifiers, and particularly for analog-input Class-D amplifiers.